1. Field of the Invention
This invention relates to secondary and tertiary methods of oil recovery and, more particularly, to improved methods for determining the progress and shape of a flood front when oil is recovered by flooding a formation.
2. The Prior Art
In oil production, primary drilling and pumping operations are frequently ineffective to recover a substantial proportion of the available oil, often leaving as much as 30 to 70% of the oil as residual. It is common, therefore, to employ so-called secondary or tertiary methods to obtain the additional oil. One such secondary or tertiary method involves flooding the producing formation with an oil-displacement fluid, such as water, steam, gases, etc., through one or more injection wells spaced from the producing well. As the leading edge, or front, of the flood fluid progresses through the formation, the oil in the formation is pushed towards the producing well. Where plural injection wells are used, the fluids from neighboring wells may merge to form a combined front, and such combined front may indeed completely surround a producing well.
It is important in maximizing the amount of oil recovered to be able to determine the direction and speed of movement of the flood front through the producing formation. Typically, however, a flood front does not progress uniformly from the injection well or wells to the producing well because the formations are usually not uniform. This non-uniformity is generally referred to as "fingering." For example, a flood front may follow a crevice in the formation and a "finger" of the flood front may "breakthrough" into the producing well, thus interrupting the production of oil. If it is known that only "fingering" has occurred and that the front has not reached the producing well, appropriate steps may be taken to prevent premature breakthrough. It is important, therefore, to know not only the location and time of arrival of the foremost edge of the flood front but also to have information of the movement and shape of the front as a whole. That is to say, for maximum oil production a complete description of the spatial shape, or "profile", of the front in the vicinity of the producing well is required.
Since oil-bearing formations differ significantly in matrix and fluid composition, it is desirable that the flood front detection process be carried out in a way which allows of the use of a wide variety of tracer elements and detection techniques, thereby permitting detection of the front or of different parts of the front in all formations likely to be encountered. Additionally, the detection process should not cause any significant interference in the movement of the front itself and should be capable of being made at a distance from the producing well sufficient to allow for modification of the flooding operation in order to maximize production.
One prior art approach to flood front detection is disclosed in U.S. Pat. No. 3,874,451 to Jones et al., according to which observation boreholes spaced from the injection wells are used to detect the arrival of the flood front by measuring a pressure change in the boreholes. By measuring the time it takes for the front to arrive at an observation borehole and knowing the distance from it to the injection hole, the progress of the front, which is related to the oil saturation, can be determined. A disadvantage of this method is that the observation boreholes must be uncased in order to measure the pressure; hence, they disturb the flood front and affect its progress. Also, the Jones et al. method does not determine the depth at which the front reached the observation well, and thus does not permit its profile to be ascertained.
U.S. Pat Nos. 2,888,569 to S. B. Jones and 3,002,091 to F. E. Armstrong disclose two other prior art techniques for detecting the arrival of a flood front. In the Jones technique, a beta-emitting tracer (e.g. krypton 85) is injected into a formation along with a flooding gas. The arrival of the flood (gas) front at the producing well is detected in the borehole with a beta detector. In the Armstrong technique, the flood fluid includes a normally stable element which is rendered unstable by neutron irradiation. At the producing well, the flood fluid is brought to the surface, separated from the oil, and bombarded with neutrons. A gamma ray detector is used to sense the presence of the unstable tracer element in the bombarded fluid. If present, it indicates that the flood fluid has reached the producing well. In both the Jones and Armstron methods, the detection of the tracer at the producing well represents a serious disadvantage because it interferes with production. These methods, moreover, afford no information about the front until it reaches the producing well. As a result, it is too late to take effective action to maximize the production of oil by controlling the flooding operation. In addition, with the Armstrong method the depth at which the front reaches the production well is not known since the detecting step is done uphole.
The foregoing and other disadvantages of the prior art are overcome by the present invention.